The present invention relates to a cranial radiography apparatus, particularly intended for dental radiography, said apparatus comprising a first body part to which is connected a second body part, the other end of which being connected to a third body part, whose other end carries a fourth body part having an x-ray source mounted at its one end and an x-ray detector at its other end and in which apparatus said second body part and said third body part are connected to each other via a first pivot shaft and, further, said third body part and said fourth body part are connected to each other via a second pivot shaft, both of said pivot shafts being essentially parallel to each other.
The most important tasks of radiography in regard to cranial imaging of the human skull include panoramic tomography of the dental arch, radiography of the dental arch in a transverse plane, radiography of the temporo-mandibular joints and cephalometric radiography of the whole skull. These imaging tasks can be accomplished using a variety of different radiographic techniques: narrow-beam tomography, linear tomography, full-field fluoroscopy, slit fluoroscopy, tomosynthesis radiography and computerized tomography, for instance. In some radiographic apparatus configurations, these different imaging modes can be carried out in the one and the same apparatus by making certain arrangements in the apparatus prior to commencing the radiographic operation and by selecting a suitable control program for the imaging operation. One of the most important and also most demanding radiographic imaging modes in the use of radiographic equipment is panoramic tomography whose basics are next described in greater detail.
Conventional panoramic radiography apparatuses are characterized in that the x-ray source is arranged to orbit about the patient""s skull, whereby the dental arch can be imaged by means of an x-ray detector orbiting on the opposite side of the scull. The image can be formed either directly on a film or stored by means of various types of solid-state detectors such as a CCD array sensor in digital format and then displayed on a screen.
In order to obtain a sharp image from a desired object, e.g., the dental arch, the sweep velocity of the x-ray detector must be equal to the sweep velocity of the x-ray beam over the object multiplied by the magnification. Then, the undesirable structures of the object being imaged are blurred invisible. The magnification is determined by the distance between the x-ray tube focus and the film plane to the distance between the x-ray tube focus and the object.
The thickness of the sharply imaged tissue layer is linearly proportional to the distance of the center of rotation to the x-ray detector plane and inversely proportional to the magnification and the beam width. Hence, the imaging process is only related to the mutual dispositions of the focus, the object and the x-ray detector plane.
The basic equation of panoramic imaging is expressed as follows:
v1/v0=L1/L0
vO=xcfx89r, where
LO=distance from tube focus F to the point of the object being imaged
L1=distance from tube focus F to the x-ray detector plane
xcfx89=angular velocity of rotational movement about the center of rotation
r=distance of the object point being imaged from the center of rotation
v1=velocity of the image point over the film or x-ray detector plane.
The function of the rotating mechanism of a panoramic tomography apparatus is to direct the x-ray beam through the patient""s jaw at a desired angle and to keep the x-ray detector at a given distance from the object being imaged. During the rotational movement, the center of rotation is moved in order to fulfill the following criteria:
orthogonality: the x-ray beam shall be incident on the object as orthogonally as possible in order to prevent adjacent teeth from overlapping at any point of the recorded image;
constant magnification: the magnification shall be maintained constant over the entire dental arch, which requirement can be fulfilled by keeping the distance between the layer being imaged and the image plane of the x-ray detector constant over the entire swept sector of the orbital movement;
motion continuity: the center of rotation shall move monotonously without discontinuities that could cause excessive forces of acceleration and thereby problems in the image quality, and
minimization of radiation dose imposed on the patient: the projection image shall be recorded so that the patient will not be exposed to an unnecessarily large dose of radiation.
The x-ray source of the panoramic tomography apparatus and the rotating mechanism of its x-ray detector must be capable of forming a projection image that satisfies the above-stated requirements set for the recording of the projection radiograph. Furthermore, the apparatus must have a design that can be manufactured at a reasonable cost to a precision free from slack causing disturbing inaccuracies in the recording of a radiograph, such as excessive play between the different elements of the rotating/translating mechanism. Hence, said mechanism must be able to accomplish the desired orbital movement of the center of rotation in a horizontal plane and, additionally, provide vertical support to the entire apparatus so that the desired orbit can be implemented with a good accuracy.
Such an orbital movement can be achieved by virtue of different conventional rotating mechanisms. In the embodiment disclosed in applicant""s patent FI 73091 (corresponding U.S. Pat. No. 4,741,007), the orbital movement is accomplished by means of two pivot shafts placed at a constant distance from each other. This construction forms the orbital geometry of the imaging process with the help of a guide groove and an active actuator.
Another prior-art technique for producing the orbital movement is the method disclosed in patent FI 87135 (filed by Instrumentarium Oy), in which the mutual distance between the pivot shafts is made variable. Hence, the orbital movement can be generated as a combination of the rotational movement and the linear movement, thus offering a freely variable orbital geometry of the imaging process.
However, the combination of a linear movement with a rotational movement has been found problematic to implement due to the demanding accuracy requirements of the orbital movement. This is because more difficult to obtain the same accuracy for the mechanism of the orbital movement when a linear movement is involved than when a rotational movement is employed alone.
In prior-art constructions, the generation of the orbital geometry has been optimized particularly for panoramic tomography. Simultaneously, the implementation of other radiographic imaging modes has generally become clumsy if not even impossible.
It is an object of the present invention to provide a method of accomplishing a rotational movement in a manner capable of providing accurate orbital geometries at more reasonable equipment costs than those of conventional constructions and simultaneously offering a selection of orbital geometries facilitating flexible switching between different cranial imaging modes (including, e.g., panoramic tomography of the dental arch, radiography of the dental arch in transverse projections commonly known as transverse radiography, imaging of the temporomandibular joints and the entire skull) using at all times a radiographic technique most appropriate for the need (e.g., narrow-beam tomography, slice tomography, full-field fluoroscopy, slit fluoroscopy, tomosynthesis radiography or computer tomography).
It is a further nonlimiting object of the invention to provide an apparatus capable of fulfilling the above-described orthogonality requirement with a good accuracy.
It is a still further nonlimiting object of the invention to provide an apparatus capable of keeping constant magnification over the entire imaged area.
It is still another further nonlimiting object of the invention to provide an apparatus capable of offering easy modifiability for cranial radiography.
It is still a further nonlimiting object of the invention to provide an apparatus in which the arms of the orbital mechanism can be readily rotated aside in order to make patient positioning easier.
It is still another further nonlimiting object of the invention to provide an apparatus that can be reorganized into a compact configuration for easier transport and/or storage and lower transport costs.
It is still another further nonlimiting object of the invention to provide an apparatus in which the orbital mechanism can be used for robotic or manual change, fetching and return of x-ray detectors. This function may be advantageously implemented by complementing the basic construction of the apparatus with a storage device for the x-ray detectors, whereby the orbital mechanism can perform under a preset control program such a transfer of the x-ray detectors between said storage device and the radiographic apparatus proper.
To achieve these goals and others to be explained later, the invention is principally characterized in that
the fixed distance between said first and said second pivot shaft is used,
said third body part is adapted rotatable in respect to said second body part by means of an active actuator,
said fourth body part is adapted rotatable in respect to said third body part by means of an active actuator, and
the rotational movement of said fourth body part is implemented by means of program-controlled operation of said active actuators, thus forcing the x-ray source and the x-ray detector to move along a predetermined orbit, said orbit defining the desired layer to be imaged sharp from the object being radiographed in tomography.
In the present invention, the rotational movement required in the panoramic exposure is realized by means of rotational movements taking place in the principal plane of the actuated body parts. The number of the body parts can be three or four, whereby the first body part is a stationary member such as a vertical column, most advantageously a telescopic column, or alternatively, a body part or bracket suitable for mounting on a wall or a ceiling. The orbital movement can be implemented most advantageously as a combination rotational movement of two or three body parts. Obviously, a greater number of such arm combinations of rotatable body parts can be used. Then, the movements of the x-ray source and the x-ray detector are realized using separate arm combinations operating independently from each other. This type of orbital mechanism implemented as a combination of rotational movements is commonly known as a SCARA (Selective Compliance Assembly Robot Arm) mechanism. This technique of implementing a movement gives at a reasonable equipment cost an orbital movement of clearly more accurate position control than other methods and equipment known in the art. Furthermore, the SCARA mechanism offers a possibility of robotic change of different types of x-ray detectors.
According to the invention, the first and the second body parts may be connected to each other in a fixed manner, or alternatively, the joint between these body parts can be provided with a pivot shaft driven by an active actuator suitable for program-controlled robotic rotation of said second body part in respect to said first body part, most advantageously about a vertical axis of rotation. The scope of the invention further covers embodiments in which the second body part is mounted by suitable fixtures on the ceiling and/or wall of the radiography room, whereby these arrangements form the first body part defined in the specifications of the invention.
The fourth body part may be formed by a C-arm having the x-ray source mounted at one end of its horizontal part and an x-ray detector at the other end, opposite to said x-ray source. Alternatively, the C-arm may be replaced by two separate members, herein called an L-arm. Such an L-arm comprises a horizontal support part with the x-ray source or the x-ray detector, respectively, mounted thereto. The x-ray source and the x-ray detector, respectively, are mounted so as to permit their free rotation about the vertical axis of the L-arm. The rotational movement is implemented with the help of an active actuator.
According to the invention, the second body part may alternatively comprise two superimposed body parts, both being connected at their one ends to said first body part and having L-arms connected to their distal ends. Such an arm combination may be utilized for the transfer of detachable x-ray detectors between the different parts of the apparatus.
During patient positioning, the arm system of the apparatus can be kept turned aside, whereby an unobstructed access of the patient to the radiography equipment is assured and the positioning of the patient becomes effortless. At the start of the radiographic exposure, the arm system is rotated to above the patient.
The auxiliary arm used in cephalographic exposures may have a shorter length than in conventional equipment, since the arm system of the present panoramic apparatus can be extended efficiently to the limits of the arm movements.